In mammals, the genetic content of a normal cell is made up of pairs of autosomal chromosomes (designated by number) plus two sex chromosomes (designated by the letters “X” and “Y”).
The total number of autosomal chromosomes varies slightly from species to species. For example, a normal cell in the human body contains a total of 44 autosomal chromosomes, in the form of pairs of chromosomes 1 through 22. In contrast, a normal cell in a mouse has a total of 38 autosomal chromosomes (in the form of pairs of chromosomes 1 through 19) while a normal cell in a horse has a total of 62 autosomal chromosomes (in the form of pairs of chromosomes 1 through 31).
For all mammals, the total number of sex chromosomes in a normal cell is 2. Normal female mammals have two X chromosomes in each cell, while normal male mammals have one X chromosome and one Y chromosome in each cell.
A variety of techniques are well known in the art for the generation of mammalian embryos in vitro, including in vitro fertilization (IVF) and nuclear transfer. Such techniques are routinely used to generate viable embryos which are transferred to the uterus of a recipient female for gestation and birth. In particular, in vitro fertilization (IVF) is routinely applied in clinical settings to allow otherwise infertile women to become pregnant and carry a baby to term.
For in vitro fertilization (IVF), oocytes retrieved from a female are fertilized using sperm retrieved from a male (for example, by simple mixing of oocyte and sperm or by intracytoplasmic injection of sperm into the oocyte), and embryonic development is initiated in vitro.
For nuclear transfer, the nucleus of a donor cell (e.g., a somatic cell) is transferred into an enucleated oocyte, for example, by cell-fusion between an enucleated oocyte and a nucleus donor cell. Following nuclear transfer the oocyte is activated to stimulate embryonic development. In other nuclear transfer methods, the nucleus of a donor cell (e.g., a somatic cell) is transferred into an enucleated fertilized oocyte, without the need for subsequent activation of the oocyte.
In such method following in vitro generation of the embryo, the developing embryo is maintained in in vitro culture, generally until the 6 or 8 cell stage, and then the embryo is transferred to the uterus of a recipient female for implantation and further development.
Preimplantation genetic diagnosis (PGD) is a technique used to provide genetic information about embryos generated in vitro prior to transfer of the embryo to a recipient female for pregnancy. For example, PGD has been applied to in vitro embryos to identify aneuploidy of autosomal chromosomes and sex chromosomes, to determine the sex of an embryo, and for the diagnosis of a variety of genetic diseases (see, for example, Ogilvie et al. J Histochem Cytochem 2005; 53:255-260 and Sermon. Human Reprod Update 2002; 8:11-20).
Prior to the development of PGD technology, the sex of a developing embryo could only be determined after the establishment of an in vivo pregnancy. Similarly, genetic diseases and conditions could only be diagnosed after the establishment of pregnancy. Thus, in human clinical settings where a pregnancy contained a child affected with a genetic disease or condition, the couple had to decide between termination of the pregnancy or delivery of an affected child whose quality of life could be severely hindered by the genetic condition.
For PGD, in vitro generated embryos are cultured under carefully controlled conditions until they reach a stage in which six to eight discrete cells are formed. In the case of IVF procedures performed on humans, this stage is usually attained three days following fertilization of the oocyte. On the day of diagnosis, the embryo is positioned by delicate micromanipulators so that a single cell can be extracted (biopsied) and separated from the remaining cells which are left intact. The genetic content of the biopsied blastomere is then assessed, for example, by polymerase chain reaction or by Fluorescence In-Situ Hybridization (FISH). Using these techniques, the number and type of chromosomes present can be determined and the health of the embryo deduced. Where an embryo of a particular sex is desired, such embryos may be identified for consideration for transfer to the recipient female. Similarly, where a genetic abnormality is detected, the particular embryo is removed from consideration for transfer to the recipient female.
For example, for FISH-based PGD of human embryos, probes are currently available to analyze up to 9 different chromosomes including the autosomal chromosomes 13, 15, 16, 17, 18, 21, and 22 and both (X and Y) sex chromosomes. Thus, FISH-based PGD may be used to determine the sex of an embryo prior to transfer. Similarly, FISH-based PGD may be used to diagnose Down's syndrome, a condition in which three copies of chromosome 21 are present.
However, removal of one or more blastomeres from the developing embryo for the purpose of performing PGD can decrease embryo viability, resulting in reduced rates of survival and continued development of embryos prior to implantation, and to reduced rates of viable pregnancy upon transfer to the recipient female. Furthermore, in certain cases, the genetic content of a single blastomere may not be representative of the embryo as a whole (e.g., where the developing embryo is a mosaic, i.e., where the genetic content of all cells is not identical). In such cases, PGD may provide an inaccurate diagnosis. For example, if the biopsied blastomere has a normal chromosomal content, while some or all of the remaining blastomeres have a chromosome abnormality, the embryo could be inaccurately identified as chromosomally normal.
Therefore, a need exists in the art for methods to assess the genetic content of an in vitro embryo, which are non-invasive (i.e., do not damage the embryo) and which accurately reflect the genetic content of the embryo as a whole. In particular, a need exists in the art for non-invasive and accurate methods to determine the sex of an in vitro embryo prior to transfer to a recipient female.
The present invention fulfills this need in the art by providing a simple, non-noninvasive method for determining the genotype of an embryo in vitro, particular for identifying the sex of the embryo, by detection of a protein associated with the genotype in a sample of culture medium obtained from a culture medium containing the embryo. This method is also broadly applicable to the diagnosis of genetic conditions and diseases, by detection of proteins associated with a genetic disease or condition in culture medium containing the embryo. This method may be used to determine the genotype of the embryo prior to implantation without adversely affecting embryo viability.